An HST/ACS investigation of the spatial and chemical structure and sub-structure of NGC 891, a Milky Way analogue

TL;DR

This study uses deep HST/ACS imaging to analyze the structure and chemical composition of NGC 891, revealing a thick disk, a spheroid with a flattened halo, and evidence of multiple stellar sub-populations indicating complex accretion history.

Contribution

First detailed structural and chemical analysis of NGC 891 using star counts, identifying a thick disk, spheroid, and chemically diverse halo with sub-structures.

Findings

Detection of a thick disk with scale height 1.44 kpc

Identification of a spheroid with a de Vaucouleurs profile

Evidence of multiple sub-populations in the halo

Abstract

We present a structural analysis of NGC891, an edge-on galaxy that has long been considered to be an analogue of the Milky Way. Using starcounts derived from deep HST/ACS images, we detect the presence of a thick disk component in this galaxy with vertical scale height 1.44+/-0.03 kpc and radial scale length 4.8+/-0.1 kpc, only slightly longer than that of the thin disk. A stellar spheroid with a de Vaucouleurs-like profile is detected from a radial distance of 0.5 kpc to the edge of the survey at 25 kpc; the structure appears to become more flattened with distance, reaching q = 0.50 in the outermost halo region probed. The halo inside of 15 kpc is moderately metal-rich (median [Fe/H] ~ -1.1) and approximately uniform in median metallicity. Beyond that distance a modest chemical gradient is detected, with the median reaching [Fe/H] ~ -1.3 at 20 kpc. We find evidence for subtle, but very significant, small-scale variations in the median colour and density over the halo survey area. We argue that the colour variations are unlikely to be due to internal extinction or foreground extinction, and reflect instead variations in the stellar metallicity. Their presence suggests a startling conclusion: that the halo of this galaxy is composed of a large number of incompletely-mixed sub-populations, testifying to its origin in a deluge of small accretions.